Investigation of Electrical Instabilities and Interface Charge in Zinc Oxide Thin-Film Transistors with High-k Dielectrics.

Abstract

Exhibiting high electron mobility compared to amorphous Silicon, transparency in the visible spectrum, and compatibility with large area and flexible substrate applications, the semiconductor Zinc Oxide (ZnO) has become a major focus of research. This work investigates insertion of high dielectric constant (high-k) insulators into ZnO thin film transistors (TFTs) in an effort to reduce the threshold voltage and increase performance for next-generation devices.

Lead Zirconate Titanate (PZT), Barium Strontium Titanate (BST), Aluminum Oxide (Al2O3), and Hafnium Oxide (HfO2) were selected for high-k investigation. Metal - Insulator - ZnO capacitors revealed that each material’s high-k qualities can be integrated with ZnO, but PZT and BST show poor gate leakage characteristics, possibly due to small conduction band offsets with ZnO. Al2O3 and HfO2 emerged as the most robust materials, however, each device exhibited clockwise hysteresis, which is indicative of interface charge. Devices also exhibited translational instability with respect to gate voltage. TFTs were fabricated with ZnO/high-k film stacks. In each case, the high-k dielectric effectively reduced the dielectric equivalent oxide thickness and thus threshold voltage. HfO2 emerged as the best candidate for ZnO/high-k devices.

Admittance Spectroscopy was found to be the most comprehensive technique to measure interface charge density in ZnO/HfO2 films and values are reported in this work. Bias - Temperature - Instabilities were investigated where positive bias temperature stress instability (PBTI) analyses reveal the dominant instability mechanism is carrier injection into the HfO2. A new method was presented to characterize PBTI threshold voltage instabilities and extract dielectric charge trap density. Negative bias temperature stress analysis shows the governing instability mechanism is charge state creation, likely, in the ZnO semiconductor. TFT stability under illumination was investigated. An increase in TFT off-current , subthreshold slope, a negative translation of threshold voltage, and an initial increase in electron mobility can be explained via a model for polycrystalline TFT drain current via thermionic emission over grain barriers where illumination causes a reduction in grain boundary charge and an increase in carriers due to photo-generation.